In recent years, for machine tool bearings (in particular, machine tool spindle bearings), minimal quantity lubrication methods such as oil lubrication, e.g., oil-air or oil-mist lubrication, and grease lubrication are used to reduce a temperature rise due to stirring resistance of lubricating oil. In oil lubrication, measures should be taken in view of a cost increase due to use of air, environmental considerations with regard to wind noise due to rolling elements cutting the air injected from a nozzle into a bearing interior and passing through the bearing interior, and outflow of used oil through an oil discharging hole, and also working environment considerations with regard to impact on human bodies, deterioration of visibility and the like due to floating of oil mists.
In light of such backgrounds, such as energy saving and reduction of environmental impact, for machine tool spindle bearings, there are increasing needs for a grease lubrication method in which there is no usage of air or outflow of lubricating oil.
Bearings are generally packaged during storage or transport, and are unpacked and charged with grease immediately before use. To prevent rusting during storage or transport, the bearings are packaged in a state in which antirust oil is applied to the entire surface of the bearing. Accordingly, if the bearing is charged with grease directly after the unpacking, the grease does not firmly adhere to a surface to be lubricated due to the antirust oil, and the grease may leak out at an early stage during an initial running-in period or in a subsequent operation, resulting in a grease life being reduced.
In particular in the case of machine tools, the maximum dmn value of a bearing during its operation is high, at least 500,000, and low temperature rise characteristics are required. Accordingly, to prevent stirring resistance due to excessive lubricant, a grease charge amount is reduced as compared with bearings used in general applications, such as motor bearings. A charging amount of grease in a bearing for a machine tool is 10 vol % to 20 vol %, sometimes 10 vol % to 15 vol % of a bearing interior space. Therefore, if a bearing for a machine tool is charged with grease directly after the unpacking as described above, the grease leaks out and the amount of residual grease becomes extremely small.
In addition, in grease lubrication, lubricating oil inside the bearing is not exchanged during use. Accordingly, if foreign substances enter the bearing interior during packaging, the foreign substances remain and hinder a rotation.
Therefore, after the unpacking and before charging with grease, it is necessary to clean and degrease bearings to remove the adhered antirust oil and foreign substances inside the bearings, causing a burden when mounting the bearings. Thus, the applicant proposes, in Patent Document 1, applying antirust oil on a surface of a bearing at a film thickness of 40 μm or less, packaging the entire bearing with a volatile corrosion inhibitor film, and keeping a pressure-reduced condition between the volatile corrosion inhibitor film and the bearing. Because an adhered amount of the antirust oil is 40 μm or less, effects on grease can be suppressed even if the grease is provided without undergoing cleaning and degreasing after the unpacking. Also, due to an airtight packaging by the volatile corrosion inhibitor film, a sufficient antirust effect can be obtained even if the adhered amount of antirust oil is reduced to 40 μm or less.